Жестко задана в коде. Например, тут arch/arm/mach-at91/board-sam9260ek.c
Код
static void __init ek_map_io(void)
53{
54 /* Initialize processor: 18.432 MHz crystal */
55 at91sam9260_initialize(18432000);
56
57 /* DBGU on ttyS0. (Rx & Tx only) */
58 at91_register_uart(0, 0, 0);
59
60 /* USART0 on ttyS1. (Rx, Tx, CTS, RTS, DTR, DSR, DCD, RI) */
61 at91_register_uart(AT91SAM9260_ID_US0, 1, ATMEL_UART_CTS | ATMEL_UART_RTS
62 | ATMEL_UART_DTR | ATMEL_UART_DSR | ATMEL_UART_DCD
63 | ATMEL_UART_RI);
64
65 /* USART1 on ttyS2. (Rx, Tx, RTS, CTS) */
66 at91_register_uart(AT91SAM9260_ID_US1, 2, ATMEL_UART_CTS | ATMEL_UART_RTS);
67
68 /* set serial console to ttyS0 (ie, DBGU) */
69 at91_set_serial_console(0);
70}